1. Field of the Invention
This invention relates to the protection of superalloys to be used at elevated temperatures, and, more particularly, to coatings applied to such superalloys, components thereof, and methods related to providing such coatings.
2. Description of the Related Art
One of the most demanding materials applications in current technology is found in the hot-stage components used in aircraft jet engines. The higher the operating temperature of an engine, the greater its efficiency, and the more power it can produce from each gallon of fuel. There is therefore an incentive to operate such engines at as high a temperature as possible. The primary limitation on the operating temperatures of a jet engine is the materials used in the hottest regions of the engine, such as gas turbine blades and vanes.
There has been much research to develop materials that can be used in high temperature engine applications. The currently most popular and successful of such materials are the nickel-base superalloys, which are alloys of nickel with additions of a number of other elements such as, for example, chromium, cobalt, aluminum, and tantalum. The compositions of these superalloys are carefully engineered to maintain their strength and other mechanical properties even during use at the high temperature of engine operation, which is in the neighborhood of 2000.degree. F. or more.
The materials used in the jet engines must operate at high temperatures, but additionally are subjected to oxidative and corrosive conditions. Oxidation of materials such as nickel and many of its alloys is rapid at engine operating temperatures. The engine components are also subjected to corrosive attack by chemicals in the burned fuel, as well as ingested agents such as salt that might be drawn into the engine as it operates near an ocean. The materials that have the best mechanical properties at high temperatures often are not as resistant to oxidation and corrosion as other materials, and there is an ongoing search for materials that offer a compromise between the best mechanical properties and the best oxidation and corrosion resistance.
High operating temperature capability can also be achieved by other techniques not related directly to the alloy compositions used in the components. For example, control of grain structures and preparation of components as single crystals may result in improved properties. Cooling passages may be provided in the components, and cooling air passed through them to lower their actual operating temperature.
The coating must be adherent to the superalloy substrate and must remain adherent through many cycles of heating to the operating temperature and then cooling back to a lower temperature when the engine is idling or turned off. Because materials of different compositions have different coefficients of thermal expansion, cycles of heating and cooling tend to cause the coating to crack and/or spall off, which results in the exposure of the superalloy substrate to the environment, and subsequent deterioration of the substrate.
Thermal barrier coatings have typically been applied to such substrates, but there has been a general desire to improve the thermal shock resistance, erosion resistance, weight, surface emissivity, heat transfer coefficient, adherence to superalloy substrates and/or compatibility with nickel based superalloys.